Message Passing Framework for Audio/Video Streaming in a Topology of Devices

ABSTRACT

Resources may be managed in a topology for audio/video streaming. The topology includes audio/video sources and sinks and intervening branch devices. Messages between these sources, sinks, and branch devices may be used for resource management.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims priority to non-provisionalapplication Ser. No. 12/821,306 filed Jun. 23, 2010 and provisionalapplication Ser. No. 61/298,936, filed on Jan. 28, 2010, herebyincorporated by reference herein.

BACKGROUND

This relates generally to devices that source and sink video and audiodata.

DisplayPort is a digital audio/video interconnect standard of the VideoElectronic Standards Association (VESA). It allows video and audio to becoupled from a computer to a video display or an audio playback system.The DisplayPort connector supports 1, 2, or 4 data pairs in a main linkthat also carries clock and optional audio signals with symbol rates of1.62, 2.7, or 5.4 gigabits per second. A 1.1 standard was approved inMay 2006 and in 2009 a 1.2 standard, with increased data rates, wasannounced. The DisplayPort 1.2 standard doubles the bandwidth of the 1.1standard.

With the DisplayPort 1.2 standard, two WQXGA monitors may sinkaudio/video data from a single source link or four WUXGA monitors maysink data from a single source link. In addition, the 1.2 standardallows a higher speed AUX which may be used for Universal Serial Bus(USB) peripheral device data transfer, microphone audio transfer, orcamera video transfer, to mention a few applications.

Display or sink devices can be connected to source devices, such aspersonal computers or consumer electronic devices, either directly orthrough what are called branch devices. Many types of branch devicesexist including repeaters that repeat audio or video information,converters that convert audio or video information from one format toanother, replicaters, which reproduce the data, and concentrators thattake streams from two or more source devices as inputs and transmit themon its downstream links. Interface standards, such as DisplayPort 1.2,allow multiple streams on one link; in such cases, these two or moreinput streams may be transmitted onto a single downstream link. Someconcentrators may operate in a switched fashion, i.e. only one selectedsource may transmit at a time.

Together, the source, sink, and branch devices form a topology in whicha given source may be streaming video to one or more sinks through zeroor more branch devices. Active video data flows through links connectingvarious device types. Each link is constrained by its bandwidth and thenumber of streams that it supports. A sink will have a limited number ofaudio and video endpoints to render the stream. Thus, based on thetopology, there may be contention for the available video or audioresources.

One such topology, shown in FIG. 1, may include two sources and fivesinks, as indicated. A source number 1 wants to stream video to sinknumber 1 and source number 2 wants to stream video to sink number 2, thelink between branch number 2 and branch number 3 is common to bothpaths. Thus, issues may arise at the source with respect to thiscontention, including how much bandwidth is available in that or anyother link along the path. Another issue is how resources on the pathcan be reserved. Still another issue is how many audio/visual streamscan be driven. Other issues include how access to shared resources canbe managed and how errors can be communicated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a audio/video distribution topologyin accordance with one embodiment;

FIG. 2 is a sequence chart for enumeration, commit and release inaccordance with one embodiment;

FIG. 3 is a schematic depiction for one embodiment of a branch device;

FIG. 4 is a flow chart for enumeration software in accordance with oneembodiment;

FIG. 5 is a message sequence chart in accordance with one embodiment;

FIG. 6 is a sequence chart for enumeration of path resources inaccordance with one embodiment;

FIG. 7 is a sequence chart showing how various display configurationscan be established for the topology shown in FIG. 6;

FIG. 8 is a depiction of a potential mapping between two sources and twosinks in accordance with one embodiment;

FIG. 9 is a flow chart for one embodiment;

FIG. 10 is a flow chart for one embodiment;

FIG. 11 shows a message sequence in accordance with one embodiment;

FIG. 12 is an up action path message sequence in accordance with oneembodiment;

FIG. 13 is a mapping of destination sequences in accordance with oneembodiment;

FIG. 14 is a message sequence chart for an uplink action path message inaccordance with one embodiment;

FIG. 15 is a depiction of connections between source and branch devicesin accordance with one embodiment;

FIG. 16 is a depiction of multiple source and branch devices accordingto one embodiment;

FIG. 17 is a depiction of a topology with two video end points inaccordance with one embodiment; and

FIG. 18 is a flow chart for one embodiment.

DETAILED DESCRIPTION

In accordance with some embodiments, specific messages may be exchangedbetween devices that source and sink video and audio data. Coordinatedaction may be taken by devices along a path between source and sinkdevices in response to those messages. Messages may be sent to atargeted destination that is specified by its address. The messagesENUM_PATH_RESOURCES, COMMIT_PATH_RESOURCES, andRELEASE_PATH_RESOURCES_may be used, as indicated in FIG. 2. Thesemessages can be sent on the AUX channel in the DisplayPortspecification, for example.

Address spaces are generated before message transmission. Each source 10sends ENUM_PATH_RESOURCE message 18 to the desired sink 16 to enumeratethe main link bandwidth and the number of streams. The branch device 14,just upstream of the desired sink 16, responds with the availablebandwidth (BW=x) and number of streams (# STREAMS=s), as indicated at20. Before this reply is propagated further upstream, the upstreambranch 12 alters the available bandwidth (BW=x′) and stream number (#STREAMS=s′) from the downstream branch 14 to reflect what is achievablefrom the downstream path, as indicated at 22.

Eventually, the source 10 gets the path resources. The message is senton a control bus (such as AUX on DisplayPort), but the query is for mainlink resources. There is no reservation of bandwidth on the control busand control messages are exchanged between devices in the topology, evenas the main link resource are completely spoken for by one or twosources.

As part of the processing of the message, each device may need to trainthe main link along the specified path to determine the amount ofbandwidth available as downstream link. Audio resources are alsoenumerated as part of this procedure. This is to determine the number ofend points that are available for streaming in any given point in time.

Link bandwidth enumeration feeds into operating system operations, suchas video mode enumeration. Based on this and asynchronous selection bythe end user of the video mode to be driven out, a commit procedure maybe accomplished using a COMMIT_PATH_RESOURCES message 24 as follows.Enumerated bandwidth may not be available at commit time. For example,different sources may send different ENUM_PATH_RESOURCES messages at anygiven time to the same sink. They could also be for different sinks withpaths that have a common link. As an example, the sequence of sourcenumber 1 enumeration, followed by source number 2 enumeration, followedby source number 1 commit, is followed, for example, by source number 2commit which fails because of the previous source number 1 commit.

The source 10 sends a COMMIT_PATH_RESOURCE message 24 to the sink 16.The message has the desired bandwidth and number of streams. All devicesalong the specified path (e.g. branches 12 and 14) reserve the resourcesfor this source. The replies 26 and 28 may indicate a success orfailure.

Each device propagates COMMIT_PATH_RESOURCES 24 only when it is able tosuccessfully commit the desired resources. In addition to independentresource commits from different source devices, it is possible that theintermediate links along the path may retrain to a lower bandwidth,providing another reason for failure. Link training is the handshakethat is performed to make the transmitter or receiver agree on anelectrical configuration. To account for a topology of devices, thisnotion is extended to an entire path, where each link on the path needsto be trained in a coordinated fashion called path training.

It is possible that some devices may have successfully committedresources for a video stream before a downstream device fails to commit.In order to release these resources, the source device may send out aRELEASE_PATH_RESOURCES_30 message after receiving a failure onCOMMIT_PATH_RESOURCES.

An active video starts after successful completion ofCOMMIT_PATH_RESOURCES. Conversely, when the stream is to be terminated,the source issues RELEASE_PATH_RESOURCES 32 to enable release ofcommitted resources at devices along the path.

Referring to FIG. 3, the source 10, sink 16, and each of the branchdevices 12 or 14, indicated in FIG. 3 as 34, includes a processor 36.The processor may be coupled to a receiver 38 and the transmitter 40.The processor 36 may also be coupled to a storage 42 which storessoftware including the enumerate software 44, in one embodiment. Thus,the storage 42 may a computer readable medium that stores instructionsexecuted by the processor 36. The storage 42 may be a semiconductor,optical, or magnetic memory.

The enumerate sequence 44, shown in FIG. 4, may be software in oneembodiment, but it also may be implemented in hardware or firmware. Acheck at diamond 46 determines whether an enumerate message has beenreceived by a branch device. If so, the message receiving branch devicereplies to the upstream branch device with the receiving device'savailable bandwidth and available number of streams.

If a receiving device does not receive the enumerate message after aperiod of time, a check at diamond 50 determines if a non-receivingdevice is an upstream device that receives a message from a downstreamdevice specifying a bandwidth and number of streams. If so, the upstreamdevice modifies the received bandwidth and number of streams needed toreflect its capabilities. It then sends either the original bandwidthand number of streams or the modified numbers, as needed, to the nextbranch or to the source, as indicated in block 54.

Referring to FIG. 5, a message sequence chart for addition and deletionof a stream includes a source 60 and two sinks 62, 64 with appropriatestream identifiers. The identifier for sink1 62 is “1” and theidentifier for sink2 64 is 1.2. Each device includes a port labeled “1”or “2”.

Then, referring to FIG. 6, a sequence chart between a source1 60, abranch plus sink1 62, and a sink2 64 is depicted. AUX refers to thecontrol channel and main link refers to the data channel.

Each link in a topology, such as that shown in FIG. 5, for example, mayconsist of independent control and data channels and the connections arepoint-to-point. There is an ability to send messages on the controlchannel to any device using addressing and routing mechanisms. Theprocedure involves locally unique identifiers for the stream at thesources and maintenance of mapping tables and concentrators, as shown inFIG. 6.

During an address generation phase, addresses are agreed to for each ofthe devices in the topology by sending address generation messages 66.Then the source sends ENUM_PATH_RESOURCES message 68 to the branch+sink162 over the control channel, indicated as AUX. It also sends aCOMMIT_PATH_RESOURCES message 70 over the control path as well as to thebranch+sink1 62.

Binding is the procedure by which devices in the topology agree on adestination for the next stream. The binding procedure begins, afterenumeration, with a source wanting to transmit a new stream, sending outan ADD_STREAM message 72 to the desired destination sink deviceidentified with a locally unique stream identifier (e.g. 1.2 for sink2).All devices along the path from source 60 to sink device 64 remember thestream identifier and the input port (e.g. 1 or 2) on which the streamhas been received in their mapping tables.

Each branch device 62 performs a mapping of the input stream identifier(ID 1 for itself) to an output stream identifier (1.2 for sink2 64). Inthe absence of multiple sources, the input stream identifier is the sameas the output stream identifier. Each branching device remembers theoutput stream identifier and output port number in its mapping table aswell.

Finally, the branch devices forward the message onwards to thedestination as indicated by the route/address contained in the message,assuming there are no other resource constraints on those devices, asindicated at 74. In case of such resource constraints, the branch devicesimply sends a negative acknowledgement to the source. The message endsat the desired destination. If the sink device is able to receive thestream, it responds with an acknowledgement 76 to the source. Otherwise,the sink device sends a negative acknowledgement. Then the sink knowsthat it needs to consume the next new stream that is on the datachannel. All branch devices propagate the acknowledgements 78 back up tothe source.

Upon receipt of the acknowledgement, the source device sends the newstream 80 out on the data channel on its link that leads to the desireddestination. The branch device routes the stream along the path for thenew stream as remembered from their mapping tables, as indicated at 82.The sink device knows it needs to consume the new stream based on themessage it had previously received and presents the stream on thedisplay.

Unbinding or deletion is performed through the delete stream message 84,86 sent to the intended destination with the same stream identifier.This causes the sink device to expect stoppage of the stream and for thebranch devices to alter their mapping tables accordingly. Receipt of anacknowledge message to a delete stream message triggers the source tostop sending the stream on the data channel.

In FIG. 7, various display configurations can be established for thetopology shown in FIG. 4. A “single display” configuration is simply onedisplay device that presents audio video data. It uses the messages 72,74, 80, 82, 84, and 86 already described. The “clone mode” configurationis where the same content 92 is sent to be displayed on two monitors ordisplay devices. “Extended desktop” is an alternative dual displayconfiguration in which different images 94, 96 are shown on bothmonitors.

When multiple sources are present in the topology, as indicated in FIG.8, from source1 98 and source2 100, each source may issue an ADD_STREAMmessage with the same stream identifier (in this case #1) at the sametime on an overlapping path. In this case, a concentrator branch device102 that is on the overlapping path for these new streams onlypropagates an ADD_STREAM message for one source (in this case source1),while blocking the others. That is, only one new stream can be added ata time. After the unblocked source's message has been conveyed on thedata channel, an additional ADD_STREAM message is propagated for theblocked source.

In the presence of multiple input ports on a branch device 104, the nextavailable stream identifier is assigned and the branch device remembersthe input stream identifier and port number to its output streamidentifier and port number in its mapping table 108.

As one use case, a new stream may be added. The concentrator branchdevice adds a new entry to its mapping table when it sees an add streammessage with an identifier that is not active. If needed, it generates anew output identifier for that stream and uses that while propagatingthe ADD_STREAM message. A concentrator branch device may add adestination address for this identifier in its mapping table. Anotheruse case is an existing stream extended. If the same source adds asecond sink to a stream that is already active through anotherADD_STREAM message, the concentrator branch device will not add a newentry in its mapping table since the mapping it had already created isstill valid. However, the concentrator does add the second destinationaddress to its input identifier in its mapping table.

Still another use case is the removal of a sink from a stream. Theconcentrator marks the address of the sink for deletion from a list ofdestination devices when it receives a delete stream message with thatsink's address for an active identifier. Subsequently, when it receivesa delete stream acknowledgement message from the sink device, itpropagates the message back to the source using the mapping table toalter the identifier that will be recognized by the source. It thendeletes the sink's address for that stream from its mapping table. Ifthat was the last sink receiving the stream with that identifier, itdeletes the entry from its mapping table. Otherwise, if there is atleast one other sink consuming the stream with that identifier, theentry in the mapping table is not deleted.

Referring to FIG. 9, a sequence 110, in accordance with one embodimentfor implementing the binding described above, is depicted. The sequencemay be implemented in software, hardware, or firmware. In softwareembodiments, the sequence may be implemented by instructions executed bya processor, such as the processor 36 shown in FIG. 3, for a branchdevice 34. In such case, the sequence may be stored on the storage 42.

Initially, a branch device receives an ADD_STREAM message, as indicatedin block 112. It stores the STREAM_ID and the input port from thatmessage in its mapping table, as indicated in block 114. Then the branchdevice maps the input STREAM_ID to an output STREAM_ID, as indicated atblock 116. It stores the output STREAM_ID and output port number in itsmapping table, as indicated in block 118. Then it forwards the messageonwards, as indicated in block 120. Ultimately, if the message issuccessfully delivered, an acknowledgment message will be received froma downstream device and the branch device forwards the acknowledgementmessage upstream, as indicated in block 122.

In some embodiments, a message passing framework 124, shown in FIG. 10,may allow for action by all devices along a path or action only by thedestination device. Messages have identifiers, with a new identifierbeing allocated as each new message is defined. The definition of amessage includes determination of whether it is a path or a destinationmessage. Path messages are of two types, depending on the direction inwhich the action is performed, on the way down towards the sink, inwhich case it is a down action path message, or on the way back up tothe source device, in which case it is up action path message. Messagesmay be initiated by any device in the topology. Each message has adestination address and associating routing information.

The message passing framework 124, shown in FIG. 10, may be implementedin software, hardware, or firmware. For example, it may be implementedin software in the form of instructions stored on a computer readablemedium, such as the storage 42 shown in FIG. 3, in the device 34, whichmay be a branch device or sink device, for example.

In accordance with one embodiment, the sequence shown in FIG. 10 beginsby receiving a message from an upstream device, as indicated in block126. The device receiving the message may be a branch device or a sinkdevice, as two examples. The receiving device, whether it is theultimate destination or not, obtains the message definition, asindicated in block 128. Then, in diamond 130, the device checks todetermine whether an up action message is indicated by the messagedefinition. If so, it performs the actions requested in the message onreceipt of the message, as indicated in block 132.

Otherwise, it is not an up action message, a check at diamond 134determines whether it is a down action message. If it is a down actionmessage, as indicated in block 136, the actions are performed on theacknowledgement, as opposed upon the receipt of the message.

Conversely, if it is not a down action message, then, as determined indiamond 138, if it is a destination message, the action is onlyperformed if the device receiving the message is the ultimatedestination, as indicated in block 140.

The message passing framework enables devices to perform coordinatedaction on a specified path in a point-to-point topology of connectedaudio visual source, branch, and sink devices. The framework can be usedfor a variety of operations including topology discovery, addressgeneration, routing, binding and stream management, resource management,and power management.

A down action message, shown in FIG. 11, works as follows. Prior tosending a message, the source 110 performs any message specific action119 needed. The message 112 is sent only if the source action succeeds.The source device transmits a message to the destination device bysending it on a downstream port, as determined based on theaddress/routing information. Each branch device 114 or 116 or sink 118that receives the message performs action 119 as required by thatmessage type. Upon the successful completion of action at thedestination (e.g. sink 118), it responds with an acknowledgement (ACK)120. This acknowledgement is propagated back up to the source.

An up action message 122 works as depicted in FIG. 12. Here, the actions119 are done as part of the acknowledgement 120.

Destination messages work as depicted in FIG. 13. Action 119 is onlydone by the destination, in this example, the sink 118. The otherdevices in the path simply forward the message and the acknowledgement.

A use of a down action path message is illustrated in FIG. 14 for pathtraining, which is training all the links on a path. In FIG. 13, theaction 119 at each device is link training. The message used isTRAIN_LINKS_ON_PATH, although any other message could be used. In FIG.13, the message is directed at branch 116.

FIG. 15 is a message sequence chart for TRAIN_LINKS_ON_PATH whenimplemented as an up action message. Here, the actions 119 all occur aspart of the acknowledgements 120.

An interface-specific framework may enable a source device to determinethat the functions enumerated through different paths are part of thesame device. The DisplayPort Standard is an example of an “interface.”Different paths for enumeration can be: a) paths featuring differentinterface types or b) just different paths within the same interfacetype. The framework enables a device to be used in conjunction withcontainer identifier initiatives that Microsoft® Windows® and othertechnologies, such as Universal Serial bus (USB) support, and enablesdevice centric rather than function centric user interfaces forconnected devices.

The framework may include a 16 byte globally unique identifier (GUID)exposed through a set of container_ID registers (which could beDisplayPort configuration data (DPCD) in the case of DisplayPort). TheDPCD is essentially a set of registers used for status checking, commandcommunication, and providing context for an interrupt. The container_IDregisters may be supported on branch devices, composite sink devices,and any device that has multiple transports.

A sink device with a given number of video end points is expected torespond with that number of Extended Display Identification Data (EDID)structures. The EDID data structure tells the source about thecapabilities of the monitor. EDID is a VESA standard. When a sink devicehas an integrated Universal Serial Bus (USB) or hub device, the globallyunique identifier of the sink matches the globally unique identifier inthe container descriptor of that USB device or hub. All functions thatare integrated into the device advertise the same globally uniqueidentifier, regardless of the interface type through which they areaccessed. In a sink that has multiple video end points, the container_IDregisters from each address returns the same globally unique identifier.For each device in the topology, the source device reads the globallyunique identifier as part of the topology discovery process. If thedevice contains a globally unique identifier, the source device readsthe globally unique identifier to determine if the same device has beenaccessed for multiple paths or through multiple interfaces.

Otherwise, a source device infers the functions that are in the samephysical device through some interface specific means. In the case wherethe interface is a DisplayPort Standard, this can be based on therelative address (RAD) of the downstream device. When faced with atopology of devices, each device initiating communication needs togenerate an address for the destination device that is valid in thenetwork. That address is called the relative address because the addressgenerated by each device is valid but could be different from what isgenerated on another source for the same destination. Then, the sourcereads the EDID from each relative address. A globally unique identifieris generated and associated with the device as identified through EDID.This generated globally unique identifier is used with the containeridentifier framework in the operating system.

Thus, the EDID contains a unique serial number in some embodiments. Ifthis is not valid, there is a change of the same globally uniqueidentifier being associated with multiple EDIDs, resulting in poor userexperience.

Multiple connections between source and branch devices are shown in FIG.16. In this case, the source generates two addresses for the sink devicesince there are two paths to that sink. Since the source reads the sameglobally unique identifier through both paths, it is able to infer thatboth paths read the same sink device. Had the globally unique identifierbeen missing, the source reads the EDID from both paths which would bethe same, generates a globally unique identifier, and associates thatglobally unique identifier with the sink device. This identifier is thenreturned to the operating system.

FIG. 17 shows an example with two video end points. The sequence of thesource is as follows. The source again generates two addresses, one foreach video end point in the sink device. The source reads the containeridentifier for the register from each video end point address. Since thesink has two interfaces into it, the framework requires a globallyunique identifier to be present in the sink and requires that globallyunique identifier to be the same in both interfaces. The source detectsthat the globally unique identifier is the same and infers that the twovideo end points are part of the same physical device.

Referring to FIG. 18, in accordance with one embodiment, a sequence 150may be implemented by a source in the form shown in FIG. 3. In someembodiments, the sequence shown in FIG. 18 may be implemented insoftware, hardware, or firmware. In software embodiments, it may beimplemented by a sequence of instructions executed by a processor, suchas the processor 36, and stored on a storage device 42.

During an initial enumeration or topology discovery phase, an identifieris read for each device in the topology (block 152). In other words, thesource obtains the identifier for the devices in the topology. Thatidentifier can be any of the identifiers already discussed herein. Thenthe source establishes a connection to a destination downstream of thesource via a path, as indicated at block 154. Then the source comparesthe identifiers of the device in the connection path, as indicated inblock 156. If, as determined in block 158, the identifiers match, thesource concludes that the path devices with matching identifiers arepart of the same branch or sink device. Thus, the ambiguity that mayarise when two devices have the same identifier may be handled easily,in some embodiments.

References throughout this specification to “one embodiment” or “anembodiment” mean that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneimplementation encompassed within the present invention. Thus,appearances of the phrase “one embodiment” or “in an embodiment” are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be instituted inother suitable forms other than the particular embodiment illustratedand all such forms may be encompassed within the claims of the presentapplication.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is: 1-24. (canceled)
 25. A non-transitory computerreadable medium storing instructions for execution by a processor in avideo source device, the instructions to cause the processor to: send anenum_path_resources message to one or more branch devices, the messageto be sent over an AUX channel, the one or more branch devicescomprising at least part of a path between the video source and a videosink.
 26. The computer readable medium of claim 25, further comprisinginstructions to cause the processor to: receive from at least one of thebranch devices an indication of available bandwidth.
 27. The computerreadable medium of claim 26, wherein the indication of availablebandwidth comprises an indication of available bandwidth of a downstreamlink of the at least one branch device.
 28. The computer readable mediumof claim 25, wherein the message comprises a path message.
 29. Thecomputer readable medium of claim 28, wherein the path message comprisesa down action path message.
 30. The computer readable medium of claim25, wherein the message comprises a message in accordance with aDisplayPort standard.
 31. An apparatus comprising: a processor to sendan enum_path_resources message to one or more branch devices, themessage to be sent over an AUX channel, the one or more branch devicescomprising at least part of a path between the video source and a videosink.
 32. The apparatus of claim 31, wherein the processor is further toreceive from at least one of the branch devices an indication ofavailable bandwidth.
 33. The apparatus of claim 32, wherein theindication of available bandwidth comprises an indication of availablebandwidth of a downstream link of the at least one branch device. 34.The apparatus of claim 31, wherein the message comprises a path message.35. The apparatus of claim 34, wherein the path message comprises a downaction path message.
 36. The apparatus of claim 31, wherein the messagecomprises a message in accordance with a DisplayPort standard.
 37. Theapparatus of claim 31, further comprising: storage coupled to theprocessor, the storage to store the enum_path_resources message.
 38. Theapparatus of claim 31, further comprising: a display coupled to theprocessor, the display to present a video stream.
 39. A methodcomprising: sending an enum_path_resources message to one or more branchdevices, the message to be sent over an AUX channel, the one or morebranch devices comprising at least part of a path between the videosource and a video sink.
 40. The method of claim 39, further comprising:receiving from at least one of the branch devices an indication ofavailable bandwidth.
 41. The method of claim 40, wherein the indicationof available bandwidth comprises an indication of available bandwidth ofa downstream link of the at least one branch device.
 42. The method ofclaim 39, wherein the message comprises a path message.
 43. The methodof claim 42, wherein the path message comprises a down action pathmessage.
 44. The method of claim 39, wherein the message comprises amessage in accordance with a DisplayPort standard.
 45. A systemcomprising: means for sending an enum_path_resources message to one ormore branch devices, the message to be sent over an AUX channel, the oneor more branch devices comprising at least part of a path between thevideo source and a video sink.
 46. The system of claim 45, furthercomprising: means for receiving from at least one of the branch devicesan indication of available bandwidth.
 47. The system of claim 46,wherein the indication of available bandwidth comprises an indication ofavailable bandwidth of a downstream link of the at least one branchdevice.
 48. The system of claim 45, wherein the message comprises a pathmessage.
 49. The system of claim 48, wherein the path message comprisesa down action path message.
 50. The system of claim 45, wherein themessage comprises a message in accordance with a DisplayPort standard.